1. Field of the Invention
The present invention relates to dielectric-waveguide attenuators, dielectric-waveguide terminators, which are used in millimeter-wave bands, and wireless apparatuses incorporating the same.
2. Description of the Related Art
A millimeter-wave integrated circuit incorporating a non-radiative dielectric waveguide, which is hereinafter referred to as an xe2x80x9cNRD waveguidexe2x80x9d is described in the Journal of the Institute of Electronics, Information and Wireless Engineers, C-1, Vol. J73-C-I, No. 3, p.87-94 (Mar. 1990.).
In the NRD waveguide, a dielectric strip is disposed between two parallel planar conductors to form an area through which an electromagnetic wave propagates. A space between the two planar conductors on each side of the dielectric strip is structured so that in that area, the electromagnetic wave is blocked. In order to form a terminator in the NRD waveguide, as shown in the Journal cited above, a resistance film for absorbing the electromagnetic wave is disposed on the dielectric strip.
FIG. 7 is a perspective view illustrating the structure of the terminator. In this figure, upper and lower planar conductors are omitted. A dielectric strip shown in the figure is placed between the upper and lower planar conductors to form an area in which an electromagnetic wave propagates. Between upper and lower parts of the dielectric strip, obtained by splitting the dielectric strip in half, a resistance sheet and a dielectric sheet are placed. As shown in FIG. 7, parts of the resistance sheet and the dielectric sheet are tapered to perform impedance conversion of the dielectric-waveguide at the tapered sections. In addition, the resistance sheet consumes LSM01-mode energy propagating through the dielectric waveguide and thereby absorbs the electromagnetic wave. As a result, the electromagnetic wave propagating from a direction A in the figure is terminated at the location where the terminator is formed, and the electromagnetic wave is hardly reflected in the direction opposite to the direction A.
In the conventional dielectric-waveguide terminator as shown in FIG. 7, in which the tapered resistance sheet is used to perform impedance conversion, it is necessary for the tapered part to be long enough to obtain sufficiently low reflection characteristics. As a result, this creates a problem in that the overall length of the terminator is increased.
Such a dielectric-waveguide terminator, for example, may be disposed at a specified port of a circulator to form an isolator, or the terminator may be disposed at a specified port of a coupler to form a directional coupler. As mentioned above, since the overall length of the terminator is increased, in the case of a dielectric-waveguide module incorporating the isolator and the directional coupler, the overall size of the module is also increased. In this case, it may be possible to locate the terminator at a specified position so as to reduce the size of the module, but it may be difficult to do so.
In addition, forming a bend in the dielectric waveguide is also effective to reduce the size of the module. However, in this case, there is a problem in that loss is increased by mode conversion between an LSM mode and an LSE mode occurring at the bend.
In addition, a dielectric-waveguide attenuator can be formed by disposing a resistance film in the dielectric strip between the ends of the dielectric waveguide. However, in order to sufficiently suppress reflection by the resistance film, a long-tapered resistance-film pattern must be used, as in the case of the above-mentioned dielectric-waveguide terminator. As a result, the dielectric-waveguide attenuator has the same problems that occur in the dielectric-waveguide terminator.
Accordingly, it is an object of the present invention to provide a dielectric-waveguide attenuator, a dielectric-waveguide terminator, and a wireless apparatus incorporating the same, in which the dielectric waveguide has a short length in a direction in which an electromagnetic wave propagates, to reduce the overall size of the module.
To this end, according to one aspect of the present invention, there is provided a dielectric-waveguide attenuator including two substantially parallel planar conductors, a dielectric strip placed therebetween so that a dielectric waveguide is formed, a reflected-wave suppressing unit for changing line impedance of the dielectric waveguide at a plurality of discontinuous points and suppressing the reflected waves of signals occurring at the plurality of discontinuous points, wherein resistance films form at least a part of the reflected-wave suppressing unit. The resistance films are disposed on a surface defined halfway through the dielectric strip and substantially in parallel to the planar conductors, and attenuate signals propagating through the dielectric waveguide.
In the above structure, the resistance films attenuate the signals propagating through the dielectric waveguide. Furthermore, the reflected-wave suppressing unit suppresses the reflections occurring at the plurality of discontinuous parts formed by the resistance films.
In this dielectric-waveguide attenuator, the resistance films may have different widths in a direction perpendicular to the dielectric strip. Even if the resistance films are connected together, the parts thereof having different widths in the perpendicular direction may be equivalent to the above-mentioned plurality of discontinuous parts.
In addition, in the above dielectric-waveguide attenuator, the resistance films may form patterns disposed intermittently in a direction in which the dielectric strip extends. The parts where the intermittent patterns are formed may be equivalent to the plurality of discontinuous parts.
As described above, since the discontinuous line-impedance changing parts are formed by the patterns of the resistance films, attenuation of the signal propagating through the dielectric waveguide and suppression of the reflected waves are simultaneously performed.
Furthermore, in the above dielectric-waveguide attenuator, the distance between the discontinuous line-impedance changing parts may be set to be an odd multiple of substantially one fourth the wavelength of a reflected wave to be suppressed. With this arrangement, the reflected wave to be suppressed can be efficiently cancelled and satisfactorily low reflection characteristics can thereby be obtained.
Furthermore, in the above dielectric-waveguide attenuator, the discontinuous parts may be formed at three or more places, and a plurality of reflected waves having different wavelengths may be suppressed by reflected waves occurring at respective ones of the discontinuous parts. With this arrangement, the reflected waves can be suppressed over a relatively wide range.
Furthermore, in the above dielectric-waveguide attenuator, the permittivity of a substrate having the resistance-film patterns formed thereon may be higher than the permittivity of the dielectric strip. With this arrangement, a wavelength shortening effect in the substrate is increased, and further, areas occupied by the resistance-film patterns are relatively reduced so that the size of the whole structure is reduced.
According to another aspect of the present invention, there is provided a dielectric-waveguide terminator including the above dielectric-waveguide attenuator disposed near the end portion of the dielectric strip.
According to another aspect of the present invention, there is provided a wireless apparatus including one of the above dielectric-waveguide attenuator and the above dielectric-waveguide terminator. For example, the dielectric-waveguide terminator can form part of an isolator and a coupler for transmitting a millimeter-wave transmission/reception signal in a millimeter-wave radar module.
Other aspects, features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.